IRQ -- Technical Information

Interrupts on the IBM PC

The  term  IRQ  stands  for  ``interrupt  request''.  The  IBM  PC  has  16
interrupts channels built into  it.  Some are reserved for system hardware;
most are  available for cards and peripheral  devices.  The following gives
the default assignments for IRQs:

    IRQ Device
    0   System timer
    1   Keyboard controller
    2   Second IRQ controller
    3   Serial port (COM) 2
    4   COM1
    5   Line printer (LPT) 2 or LPT3
    6   Floppy-disk controller
    7   LPT1
    8   Real-time clock
    9   Re-directed IRQ2
    10  Available
    11  Available
    12  Motherboard mouse port (available if no mouse)
    13  Mathematics coprocessor
    14  Hard-disk (AT) controller
    15  Available

As you can see, there are two banks of interrupt controllers, each of which
controls  eight interrupts,  with IRQ2  latched  to the  first port  on the
second chip, IRQ9.

Channel 5  handles two  parallel ports  -- LPT2 and  LPT3.  If  you install
three serial ports onto your system, be careful on how you jumper the card,
or you will confuse your system.

Due to the  design of the PC, IRQ 7  can display spurious interrupts when a
device signals an IRQ line other than 7, then cancels the signal before the
interrupt controller figures out which line the IRQ occurred on.  Thus, you
should not assign other devices to IRQ 7, if at all possible.

Three interrupt channels are  available for user hardware: channels 10, 11,
and 15.  Channel 12 is also available if you do not have a bus mouse.

Only two  interrupts are available  for serial ports, COM1  and COM2.  Note
that COM3 and COM4 are ``linked'' to COM1 and COM2, respectively.  For this
reason, if you have both COM1  and COM3 your system, or both COM2 and COM4,
only one  of the pair can  be interrupt driven; the other  port of the pair
must be polled.

See Also

Administering COHERENT

© 2012 by Stephen A. Ness